High efficiency wastewater treatment has become increasingly important as the world's population continues to grow. The quantity of water needed for human consumption and other uses has increased at a rapid pace, while the amount of naturally available water remains unchanged. The ever-increasing demand for usable, clean water has made reclamation of wastewater an essential component of growth and development of human populations.
In the United States and other developed nations, as existing metropolitan areas become overcrowded, developers are encouraged or required to construct new housing in previously undeveloped areas. Many of these undeveloped areas lack sufficient water for consumption, irrigation and similar purposes, necessitating reclamation and reuse of available water resources. For development in these areas to be successful, sewage from the residential use of water, commonly referred to as wastewater, is therefore a primary target for reclamation.
Residential wastewater has a high water content, but requires substantial processing before it can be reused because of the human waste and other contaminants mixed with it. To achieve reclamation of residential wastewater in many new development areas, isolated from existing sewage treatment facilities, on-site wastewater treatment and reclamation is highly advantageous or essential.
A wide variety of different wastewater treatment systems have been proposed for reclaiming residential sewage and other categories of wastewater. One such system disclosed in U.S. Pat. No. 2,528,649, incorporates a simple sedimentation tank for separating solid waste, or “sludge”, from wastewater. After sedimentation, the sludge is passed to a digestion system where it is allowed to settle so that clear aqueous liquid separates from the sludge. The clarified liquid is redirected back to the sedimentation tank. Unfortunately, this system suffers from a number of shortcomings that make it inefficient. In particular, the system incorporates a relatively crude sedimentation system that merely allows the influent sewage to separate and does not aerate or facilitate processing of the sewage in any other way.
A number of wastewater treatment processes comprise “biological” systems utilizing microorganisms contained in an activated biomass, or sludge for the removal of COD, phosphorous and/or nitrogen from wastewater. These treatment processes typically incorporate multiple treatment phases or “zones”, namely: (1) a preliminary treatment area; (2) a primary treatment area; and (3) a secondary treatment area. Preliminary treatment is primarily concerned with the removal of solid inorganics from untreated wastewater. Typically, this preliminary treatment encompasses a two-stage treatment process in which the debris is removed by screens and/or settling. Organic matter is carried out in the fluid stream for subsequent treatment. Primary treatment entails a physical process wherein a portion of the organics, including suspended solids such as feces, food particles, etc. is removed by flotation or sedimentation. Secondary treatment typically encompasses a biological treatment process where microorganisms are utilized to remove remaining organics, nitrogen and phosphorous from the wastewater fluid stream. Microorganism growth and metabolic activity are exploited and controlled through the use of controlled growth conditions.
In large scale municipal or industrial applications, biological treatment processes typically utilize a basin or other reservoir in which the wastewater is mixed with a suspension of biomass/sludge. Subsequent growth and metabolism of the microorganisms, and the resultant treatment of the wastewater, is carried out under aerobic and/or anaerobic/anoxic conditions. In most large scale municipal or industrial treatment systems, the various components of the treatment process are performed in discrete basins or reactors. As such, there is a continuous flow of the wastewater from one process step to the next. Biomass containing the active microorganisms may be recycled from one process step to another. The conditioning of such biomass to enhance growth of particularized subgroups of microorganisms possessing a proclivity for performing a specific type of metabolic process, e.g. phosphate removal, nitrogen removal has been the subject matter of numerous patents, including: U.S. Pat. No. 4,056,465; U.S. Pat. No. 4,487,697; U.S. Pat. No. 4,568,462; U.S. Pat. No. 5,344,562. The optimization of other components or aspects of biological wastewater treatment has also engendered a variety of patents, including: U.S. Pat. No. 2,788,127; U.S. Pat. No. 2,875,151; U.S. Pat. No. 3,440,669; U.S. Pat. No. 3,543,294; U.S. Pat. No. 4,522,722; U.S. Pat. No. 4,824,572; U.S. Pat. No. 5,290,435; U.S. Pat. No. 5,354,471; U.S. Pat. No. 5,395,527; U.S. Pat. No. 5,480,548; U.S. Pat. No. 4,259,182; U.S. Pat. No. 4,780,208; U.S. Pat. No. 5,252,214; U.S. Pat. No. 5,022,993; U.S. Pat. No. 5,342,522; U.S. Pat. No. 3,957,632; U.S. Pat. No. 5,098,572; U.S. Pat. No. 5,290,451; Canadian Patent # 1,064,169; Canadian Patent # 1,096,976; Canadian Patent # 1,198,837; Canadian Patent # 1,304,839; Canadian Patent # 1,307,059; Canadian Patent # 2,041,329.
Biological removal of organic carbon, nitrogen and phosphorus compounds from waste water requires attention to special environmental conditions within the processing equipment. For instance, for bacteria and other microbes to convert organic carbon compounds (measured as BOD) to carbon dioxide and water, a well mixed aerobic environment is required. Approximately one pound of oxygen is required for each pound of BOD removed. To convert nitrogen compounds to nitrogen gas and carbon dioxide, nitrosomas and nitrobacter operate in an aerobic environment consuming inorganic carbon. Approximately 4.6 pounds of oxygen is required for each pound of ammonia-N converted to nitrate-N (assuming alkalinity is sufficient). Subsequently, facultative bacteria operate in an anoxic environment consuming organic carbon and liberating nitrogen gas. Approximately 2.6 pounds of oxygen is recovered for each pound of nitrate-N converted to nitrogen gas. To biologically tie up phosphate in the cell mass, an anaerobic step to produce volatile fatty acids is required. This is followed by Poly P microbes consuming large amounts of phosphorus required to metabolize the volatile fatty acids in an aerobic environment thus concentrating the phosphate in the biomass (see, e.g., Abstract by Dr. W. Wilson Western Canada Water and Wastewater Conference Calgary AB. January 2002.)
The combination of these many biological processes ideally results in a Biological Nutrient Removal (BNR) process, sometimes called tertiary treatment. However, a well-designed tertiary treatment operation requires coordination and sequencing of a complex assemblage of components, processes and conditions. Each of the constituent biological processing steps proceeds at its own rate, with specific environmental parameters required. Efficient tertiary processing also requires the correct amounts of specialty microbes to sustain the microbial populations and perform specific processing functions.
Current wastewater treatment systems which attempt to provide tertiary treatment include Upflow Sludge Bed Filter (USBF), Sequencing Batch Reactor (SBR) and Membrane Separation Activated Sludge (MSAS) systems. The Sequencing Batch Reactor (SBR) process is a modification of the conventional activated sludge process. U.S. Pat. No. 5,503,748 discloses a long vertical shaft aerator applied to the SBR technology. The SBR process employs a number of discrete steps, typically comprising sequential fill, reaction, settlement and decantation of wastewater with biomass in an enclosed reactor. In the initial step of this process, wastewater is transferred into a reactor containing biomass, and combined to form a mixed liquor. In the reaction step of the treatment process the microorganisms of the biomass utilize and metabolize and/or take up the nitrogen, phosphorous and/or organic sources in the wastewater. These latter reactions may be performed under anaerobic conditions, anoxic conditions, aerobic conditions, or a combination thereof to manipulate organism growth, population dynamics and contaminant processing. The length of this stage will be dependent on the waste's characteristic, concentration of the biomass, and other factors. Following the reaction cycle, the biomass in the mixed liquor is allowed to settle out. A sludge blanket settles on the bottom of the reactor leaving a treated effluent supernatant. The treated and clarified wastewater (i.e. effluent) is subsequently decanted and discharged. The reactor vessel is then refilled and the treatment process cycle reinitiated. Thus, the sequencing batch reactor's process is based on discrete operation in time, whereas other wastewater treatment processes are based on distinct operations in space, e.g., by performance of different reactions in separate vessels.
A number of additional wastewater treatment designs feature an air-lift reactor, which is a mechanically simple, combined gas-liquid flow device characterized by fluid circulation in a defined cyclic pattern through a set of specifically designed channels. Fluid motion is due to the mean density difference in an upflow (riser) and downflow (downcomer) sections of the reactor. The air-lift reactor is ordinarily comprised of distinct zones with different flow patterns. The riser is typically the zone where the gas is injected creating a fluid density difference, resulting in upward flow of both liquid and gas phases. At the top of the reactor, there is a gas-liquid separator section, which is typically a region of horizontal fluid flow and flow reversal where gas bubbles disengage from the liquid phase. The downcomer is the zone where the gas-liquid dispersion or degassed liquid ordinarily recirculates to the riser. The downcomer zone exhibits either single-phase, two-phase cocurrent, or two-phase mixed cocurrent-countercurrent downward flow, depending on whether the liquid velocity is greater than the free-rise velocity of the bubbles. The base section at the lower end of the vessel communicates the exit of the downcomer to the entrance of the riser.
The air-lift reactor has predominantly been used for microorganism fermentation processes such as the ICI single cell protein production. Nonetheless, a number of systems are known which utilize air-lift reactors for wastewater treatment. Among these examples is the Betz reactor (Gasner, Biotech. Bioeng. 16:1179–1195, 1974), and “deep shaft” bioreactors for effluent treatment (see, e.g., Hines et al., Chem. Eng. Sym. Ser. U.K. 41:D1–D10, 1975).
Following the original development of deep shaft bioreactor technology, recent efforts have led to improvements in long vertical shaft bioreactor systems for wastewater treatment. Among these improvements, U.S. Pat. Nos. 4,279,754, 5,645,726, and 5,650,070 issued to Pollock each disclose a modified vertical shaft bioreactor system for the treatment of biodegradable wastewater and/or sludge. Generally, these vertical shaft bioreactor systems comprises a bioreactor, a solid/liquid separator and intervening apparatus in communication with the bioreactor and separator. The bioreactor comprises a circulatory system which includes two or more vertical, side-by-side or coaxial chambers, a downflow chamber (downcomer) and an upflow chamber (riser). These chambers are connected at their upper ends through a surface basin and communicate at their lower ends via a common “mix zone” adjacent the lower end of the downcomer.
In addition to the mix zone, these reactors feature a “plug flow zone” located below the mix zone and communicating therewith. As previously described, the term “plug flow” has referred to a net downward migration of solid particles from the mix zone toward an effluent outlet located at the lower end of the reactor. In one application to sludge digestion the net downward migration has been reported by Guild et al. (Proceedings WEF conf., Atlanta Ga., October 2001), to include local back mixing only, but over extended periods of operation (e.g., about 16 hours), inter-zonal mixing occurs.
The waste-containing liquor (“mixed liquor”) is driven through the circulating system (i.e., between the downflow and upflow chambers, the surface basin and the mix zone) by injection of an oxygen-containing gas, usually air, near the bottom of the reactor (e.g., at the mix zone and plug flow zone). A portion of the circulating flow is directed to the plug flow zone and is removed at the lower end thereof as effluent. In wastewater treatment reactors, the air is typically injected 5–10 feet above the bottom of the reactor and, optionally, immediately below the lower end of the downcomer. The deepest air injection point divides the plug flow zone into a quasi plug flow zone with localized back mixing above the deepest point of air injection, and a strict plug flow zone with reportedly no mixing below the deepest point of air injection.
At start-up of the bioreactor, air is injected into the riser in the nature of an air lift pump, causing liquor circulation between and through the upflow and downflow chambers. Fluid in the downcomer has a higher density than the liquid-bubble mixture of the riser and thereby provides a sufficient lifting force to maintain circulation.
Once the bioreactor circulation is thus initiated, all of the air injection is diverted to the mix zone and/or plug flow zone. The air bubbles that rise out of these zones are trained into the upflow chamber and are excluded from the downflow chamber where the downward flow of liquor exceeds the rise rate of the bubbles. Dissolved oxygen in the circulating mixed liquor is the principal reactant in the biochemical degradation of the waste. As the liquor ascends in the riser to regions of lower hydrostatic pressure, this and other dissolved gases separate and form bubbles. When the liquid/bubble mixture from the riser enters the basin, gas disengagement occurs. To facilitate this purpose, the surface basin is ordinarily fitted with a horizontal baffle at the top of the upflow chamber to force the mixed liquor to traverse a major part of the basin and release spent gas before re-entering the downflow chamber for further treatment.
U.S. Pat. No. 5,650,070 discloses a process where influent waste water is introduced at depth into the riser chamber through an upwardly directed outlet arm of an influent conduit. A zone of turbulence is created at the lower end of the downflow chamber by the turn-around velocity head as the circulating flow reverses from downward to upward flow. This mix zone is not well defined but typically is between 15–25 feet deep. A portion of the mixed liquor in the mix zone flows downwardly into the top of the plug flow zone in response to an equal amount of treated effluent being removed from the lower end of the plug flow zone into an effluent line, as discussed above. During operation of the bioreactor the flow of influent liquor to and effluent liquor from the bioreactor are controlled in response to changes in level of liquid in the connecting upper basin.
Reaction between waste, dissolved oxygen, nutrients and biomass (including an active microbial population), substantially takes place in an upper circulating zone of the bioreactor defined by the surface basin, the upflow and downflow chambers and the mix zone. The majority of the contents of the mix zone circulate upwardly into the upflow chamber. In this upflow chamber undissolved gas, mostly nitrogen, expands to help provide the gas lift necessary to drive circulation of the liquor in the upper part of the reactor. The spent gas is released from the liquor as it traverses the horizontal baffle in the surface basin. The plug flow zone located below the upper circulating zone provides a final treatment or “polish” to the mixed liquor flowing downward from the mix zone to effluent extraction at the lower end of the reactor.
The injected oxygen-containing gas dissolves readily under pressure in the liquor in the plug flow zone where there is localized back mixing resulting in a slow net downward movement of liquor. Undissolved gas (bubbles) migrate upward to the very turbulent mix zone under pressure. The gas to liquid transfer in this zone is very high, reaching overall reactor oxygen transfer efficiencies in excess of 65%. The products of the reaction are carbon dioxide and additional biomass which, in combination with unreacted solid material present in the influent wastewater, forms a sludge (or biosolids).
In addition to aerobic digestion of BOD, it is becoming more and more important to couple biological nutrient removal (BNR) of nitrogen and phosphorous compounds with conventional wastewater treatment. As the demand for higher quality liquid effluent discharges increase, the need for technologies as provided by the present invention has become increasingly more compelling. The old Secondary Biological treatment standard of 30 mg/L BOD and 30 mg/L TSS is no longer adequate in many jurisdictions and limits are now often placed on nitrogen and phosphorus as well. Effective removal of these nutrients is essential in view of existing and developing environmental laws aimed at preventing eutrophication of natural waters and the attendant ecosystem damages that result therefrom.
In basic terms, nitrogen removal is accomplished by converting ammonia contained in a mixed liquor stream to nitrites and nitrates, in the presence of oxygen, which is known as an aerobic nitrifying stage. Ammonia conversion to nitrite is carried out by microbes known as Nitrosomonas, while the conversion of nitrite to nitrate is accomplished by Nitrobacters. Nitrate conversion to nitrogen gas occurs in an anoxic dentrifying stage that takes place in a suspended growth environment devoid of dissolved oxygen. Nitrogen, carbon dioxide and water is produced, with the gas being vented from the system. Nitrification rates can be optimized by regulating interdependent waste stream parameters such as temperature, dissolved oxygen levels (D.O.), pH, solids retention time (SRT), ammonia concentration and BOD/TKN ratio (Total Kjeldahl Nitrogen, or TKN, is organic nitrogen plus the nitrogen from ammonia and ammonium). Higher temperatures and higher dissolved oxygen levels tend to promote increased nitrification rates, as does pH levels in 7.0 to 8.0 range. Sludge retention times of from 3.5 to 5, and preferably 5–8, days dramatically increase nitrification efficiency, after which time efficiencies tend to remain constant. Increases in ammonia concentration increases the nitrification rate but only to a maximum level attainable after which further ammonia concentration increases do less to increase the rate of nitrification. Rates have also been shown to be maximized at BOD/TKN ratios of less than 1.0 (see, e.g., Abstract by Dr. W. Wilson, Western Canada Water and Wastewater BNR conference Calgary AB Canada January. 2002]]
Physical/Bio-Chemical phosphorous removal typically requires an anaerobic suspended growth zone at the start of the system, and a sludge fermentation tank to supply volatile fatty acids (VFA's) for the energy needs of the phosphorous ingesting organisms (Acinetobacters). Recently it has been reported that anerobic force mains can generate sufficient volatile acids to premit substantial biological phosphorus removal.
Refractory treatment and polishing stages may be added to the process, downstream of the final clarification stage. In many waste streams, the majority of organic compounds (80%–90%) are easily biodegraded. The remaining fraction biodegrade more slowly and are termed “refractory” compounds. Prior art biological nutrient removal designs incorporate a single sludge and a single clarifier, for example, U.S. Pat. No. 3,964,998 to Barnard, but in that case the overall oxidation rate of the system has to be reduced to satisfy the slowest compound to oxidize.
Biological nutrient removal (BNR) systems can take various process configurations. One such embodiment is the five stage Modified Bardenpho™ process, which is based upon U.S. Pat. No. 3,964,998 to Barnard. It provides anaerobic, anoxic and aerobic stages for removal of phosphorous, nitrogen and organic carbon. More than 24 Bardenpho™ treatment plants are operational, with most using the five stage process as opposed to the previously designed four stage process. Most of these facilities require supplemental chemical addition to meet effluent phosphorous limits of less than 1.0 mg/L. Plants using this process employ various aeration methods, tank configurations, pumping equipment and sludge handling methods. WEF Manual of Practice No. 8, “Design of Municipal Wastewater Treatment Plants”, Vol. 2, 1991.
In the context of vertical bioreactor technology, Pollock (U.S. Pat. No. 5,651,892, issued Jul. 29, 1997, incorporated herein by reference) discloses an innovative process utilizing a vertical bioreactor linked to a flooded filter via a flotation separator. According to this design, improved reaction rates are achieved by separating the biomass into a high rate aerobic organic carbon removal step, followed by an aerobic nitrification step using a separate nitrifying biomass. These steps are then followed by a high rate denitrification step in an anoxic environment created by feeding influent and return mixed liquor or effluent into that zone to provide a source of organic carbon and consume the oxygen.
Incorporation of an anaerobic processing step for phosphate removal is typically done in a separate reactor—due to the long fermentation time required for volatile fatty acid production. Furthermore, phosphorus removal in single mixed liquor systems is difficult to implement because the phosphate rich biomass produced in the aerobic portion of the process should not contact the anaerobic fermentation reactor product due to the risk of re-solubilizing the entrapped phosphate. In other instances, biological phosphorus removal is augmented by addition of metal salts such as ferric chloride or alum. These can be added directly into the aerobic zone of the reactor to chemically bind the phosphate.
Thus, a variety of treatment systems, including coupled vertical shaft reactors and SBR's, have been successfully used to provide tertiary wastewater treatment. However, these tertiary treatment systems involve a single mixed liquor process wherein all of the specialty microbes involved in the process are mixed together. These include autotrophic organisms that utilize energy from inorganic material (e.g., the nitrifiers Nitrosomonas and Nitrobacters), and heterotrophs which utilize organic energy sources and include the aerobic BOD removers and the Acinetobacter biological phosphorous removers (Bio-P organisms). Therefore, in all of these types of systems, the rate of treatment is controlled by the slowest performing microbe, usually nitrosomas which converts ammonium to nitrite. Due to the slow overall rate of treatment, these single mixed liquor systems are called extended aeration systems and are quite energy intensive.
Despite the foregoing developments and advancements in wastewater treatment technologies, there remains an urgent need in the art for improved wastewater treatment systems that can satisfy a broadened range of uses and perform expanded and enhanced functions not satisfied by existing wastewater treatment systems. For example, there is a long unmet need in the art for a simplified wastewater treatment process and apparatus that provides enhanced biological nutrient removal (BNR) and which, in certain embodiments, can produce class A bio-solids required for unrestricted land applications. In addition, there remains an unfulfilled need for wastewater treatment systems and methods that satisfy these expanded functions while minimizing the costs and environmental impacts that attend conventional wastewater treatment plant installation and operation.
Surprisingly, the present invention satisfies these needs and fulfills additional objects and advantages which will become apparent from the following description and appended drawings.